Hydraulic pressure control valve

ABSTRACT

A hydraulic pressure control valve, wherein a valve body having six oil grooves on its outer periphery at specified intervals is rotatably disposed in a cylindrical casing having six oil grooves on its inner periphery at specified intervals, and throttles are constructed on both sides of the respective oil grooves, thereby two throttling units each of which consisting of six throttles, are constructed and respective two throttles facing to each other in the radial direction among respective six throttles constructing respective throttling units have regions in which throttle areas do not substantially change until the relative angular displacement between the valve body and the casing reaches a predetermined magnitude.

This is a continuation of application Ser. No. 07/739,948 filed Aug. 2,1991, now U.S. Pat. No. 5,133,384.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydraulic pressure control valvewhich controls pressure oil to be given to two different destinationscorresponding to a relative angular displacement between the valve andthe casing, particularly to a hydraulic pressure control valve which isused in a hydraulic power steering apparatus in order to controlpressure oil to be given to two oil chambers of a hydraulic doubleacting cylinder for steering assisting corresponding to steeringoperation.

2. Description of Related Art

There is a hydraulic power steering apparatus which generates hydraulicpressure force in a hydraulic double acting cylinder provided in asteering mechanism, corresponding to direction and strength of steeringtorque applied to a steering wheel and assists force required forsteering operation. In such a power steering apparatus, a hydraulicpressure control valve constructed in the middle of a steering shaft isdisposed between a hydraulic pump (pressure oil supply source) driven byan engine and an oil tank (discharge port) maintained at a low pressure,and two oil chambers of a hydraulic cylinder (destination). By theoperation of hydraulic pressure control valve, pressure oil to the twooil chambers is controlled corresponding to steering operation.

As a hydraulic pressure control valve, whose cross sectional view isshown in FIG. 1, is generally used. In the control valve, there areprovided a cylindrical casing 10 having a plurality of oil grooves 11,11, . . . extending in the axial direction on its inner peripheralsurface at specified intervals and a cylindrical valve body 12 axiallyand rotatably disposed in the casing 10, and having oil the same numberof grooves 13, 13 . . . as the oil grooves 11, 11 . . . , extending inthe axial direction, on its outer peripheral surface at specifiedintervals, thereby aforementioned control operation is carried outaccording to the relative angular displacement around the axis betweenthe both. The casing 10 and the valve body 12 are so arranged that theformer oil grooves 11, 11 . . . and the latter oil grooves 13, 13 . . .are stagger-arranged in the peripheral direction. Both of thecommunicating portions between the oil groove 11 and the oil grooves 13,13 in the peripheral direction adjacent on both sides of the oil groove11 function as two throttling units 2, 3 which change the areas in sucha manner that one is increased when the other is decreased according toa relative angular displacement.

In the case where the hydraulic pressure control valve is applied to apower steering apparatus, an input shaft connected with a steering wheeland an output shaft connected with a steering mechanism are coupledcoaxially through a tortion bar 4, the casing 10 being fixed coaxiallyon one joint end and the valve body 12 being integrally constructed onthe other joint end. When steering torque is applied on the steeringwheel, the relative angular displacement responsive to the tortion ofthe tortion bar 4 is generated between the casing 10 and the valve body12. And as shown in the drawing, among the oil grooves 11, 11 . . . atthe casing 10 side, half of them positioning every other groove arecommunicating with one oil chamber of a hydraulic cylinder S forassisting steering and the rest of them are communicating with other oilchamber respectively. Among the oil grooves 13, 13 . . . at the valvebody 12 side, half of them positioning every other groove arecommunicating with a discharge side of a hydraulic pump P being apressure oil supply source and the rest of them are communicating withan oil tank T being a discharge port respectively. Accordingly, both oilchambers of the hydraulic cylinder S being a destination of pressure oilare communicating with the discharge side of the hydraulic pump P or theoil tank T through one throttling unit consisting of the throttles 2, 2. . . , and are communicating with the oil tank T or the discharge sideof the oil pump P through other throttling unit consisting of thethrottles 3, 3 . . . .

In the case where the relative angular displacement is generated betweenthe casing 10 and the valve body 12 responsive to steering torqueapplied to the steering wheel for example, throttle areas of thethrottles 2, 2 . . . are increased and throttle areas of the throttles3, 3 . . . are decreased, when pressure oil introduced from thehydraulic pump P to each oil groove 13 passes through the throttles 2, 3on both sides thereof to be flown into the adjacent oil grooves 11, 11,the conduction resistance at the throttle 2 becomes smaller than that atthe throttle 3. Therefore, between the oil groove 11 communicating withthe oil groove 13 through the throttle 2 and the oil groove 11communicating with the same through the throttle 3, and between both oilchambers of the hydraulic cylinder S respectively communicating with theoil grooves 11, 11, such pressure difference that the former is higherthan the latter is generated, the hydraulic cylinder S generatingsteering assisting force corresponding to the pressure difference.

In addition, in such a hydraulic pressure control valve, the numbers ofthe oil grooves 11, 11 . . . at the casing 10 side and the oil grooves13, 13 . . . at the valve body 12 side are apt to be reduced in order torealize a small-sized valve, and as shown in FIG. 1, the one having sixoil grooves 11, 11 . . . and six oil grooves 13, 13 . . . and sixthrottles 2, 2 . . . and six throttles 3, 3 . . . provided therebetween(hereinafter to be called a six-equally spaced valve) has been put intopractical use.

By the way, the strength of the force required for steering a vehiclecorresponds to the strength of road reaction force acting upon wheels.In the case where road reaction force is large such as low speedrunning, large force is required for steering operation. On the otherhand, in the case where road reaction force is small such as high speedrunning, steering can be carried out by relatively small force.Accordingly, in a power steering apparatus, it is desired that suchsteering assisting force as follows is generated particularly in orderto heighten the straight-drive stability. In the case where steeringtorque to be applied to the steering wheel is small, almost no steeringassisting force is generated, and equal rigidity to that of handsteering is given to the steering wheel. When steering torque reaches apredetermined strength, it is required that a large steering assistingforce is generated thereby force required for steering operation is tobe as small as possible. In the middle state of the both, it is requiredthat steering assisting force is gradually increased in proportion toincrement of steering torque in order not to generate unnatural steeringfeeling by the sudden decrease of the force required for steeringoperation. Accordingly, desirable increment characteristic of steeringassisting force in a power steering apparatus is, as shown in "SAETechnical Paper No. 880706, 1988", so called two-step characteristics asshown in FIG. 2, having a proportional gradual increase region between aconstant region wherein steering torque is small and sudden increaseregion wherein steering torque is large. Further desirablecharacteristic is the one in which increasing rate (inclination of thegradual increase region in FIG. 2) more than a predetermined rate isobtained in the gradual increase region.

The hydraulic cylinder S generates steering assisting forcecorresponding to aforementioned operation of the hydraulic pressurecontrol valve, and the strength thereof depends upon the reduction ofthrottle areas generated in the throttles 2, 2 . . . or the throttles 3,3 . . . responsive to the relative angular displacement between thecasing 10 and the valve body 12. In order to obtain aforesaid two-stepcharacteristics, it is necessary to obtain the decreasing mode ofthrottle areas which gradually decreases until the relative angulardisplacement reaches a predetermined value and suddenly decreases whenit reaches the predetermined value, in the throttles 2, 2 . . . and thethrottles 3, 3 . . . . In order to realize aforesaid decreasing mode,such a hydraulic pressure control valve as represented by U.S. Pat. No.3,591,136 is well known wherein a notch portion 20 is provided at thecorner formed between the inner periphery of the casing 10 and the sidewall of the oil groove 11 and having throttles 2 and 3, which consistsof, as show in FIG. 3, a first portion 21 crossing the side wall of theoil groove 11 at substantially a right angle and almost in parallel withthe inner peripheral surface of the casing 10, and of a second portion22 crossing the projected end of the portion 21 and the inner peripheralsurface of the casing 10 at substantially a right angle to connect them.In addition, FIG. 3 is a view where the peripheral surfaces of thecasing 10 and the valve body 12 are unfolded on the straight line. Thatis to say, in the throttles 2 and 3 provided with the notch 20, untilthe corner of the valve body 12 side facing the notch portion 20substantially agrees with the projected end of the first portion 21according to the increase of the relative angular displacement, constantthrottle areas depending only upon the depth d in the radial directionof the notch 20, can be obtained thereby, also by the proportionalreduction of the throttle areas in the other throttles 2 and 3 in whichthe notch 20 is not provided, aforementioned decreasing mode can beobtained. In addition, the notch 20 can be provided at the corner of thevalve body 12 side facing the corner of the casing 10 side, however, itis extremely difficult to form the notch 20 at the valve body 12 side.Generally, the notch 20 is formed by cutting the corresponding corner ofthe casing 10 by polishing after broaching the inner peripheral surfaceof the casing 10.

Now, as aforementioned equally-spaced six valves to obtain two-stepcharacteristic, there is the one disclosed in Japanese PatentApplication Laid-Open No. 61-94870, 1986. The notches are provided atthree throttles 2, 2, 2 and the throttles 3, 3, 3 respectivelypositioning at both sides of either three oil grooves 13, 13, 13communicating with the discharge side of the hydraulic pump P or thosecommunicating with the oil tank T among six oil grooves 13, 13 . . . atthe valve body 12 side in FIG. 1.

In the two-step characteristic obtained, as shown in FIG. 2, by theformation of the notch 20, it is desired as aforementioned that aninclination more than the predetermined one is obtained at the gradualincrease region. As the inclination depends upon the sum of the depths din the radial direction of the notch 20 as shown in FIG. 3 and is apt toincrease as the sum decreases, it is necessary to make each depth d ofeach notch 20 as small as possible in order to obtain the desiredtwo-step characteristic. On the other hand, in forming the notch 20carried out by aforementioned procedure, a processing error of about0.02 mm in broaching and that of about 0.005 mm in polishing the innerperipheral surface of the casing 10 are generated respectively. Inaddition, generation of a core displacement error of about 0.01 mmcannot be prevented during broaching and polishing. The lower limitvalue of the depth d should be 0.04 mm to secure some degree of oilpassage even when these deviations are piled up in the same direction.

However, as disclosed in aforementioned Japanese Patent ApplicationLaid-Open No. 61-94870, 1986, in the case where notches 20 are providedat three throttles 2, 2 . . . and three throttles 3, 3 . . . , even whenthe respective depths d of the notches are set to the lower limit value0.04 mm, there has been a problem that the inclination in the gradualincrease region of steering assisting force is too small in a powersteering apparatus in which the hydraulic pressure control valve isused, thereby steering feeling is worsened by sudden decrease of theoperation force for steering wheel in a transit region from gradualincrease region to sudden increase region.

The applicants of the present invention has proposed in U.S. Pat. No.4,924,910 a hydraulic pressure control valve in which communication ofthe oil tank T with the two oil grooves 13, 13 facing to each otheramong the four oil grooves 13, 13 . . . to be communicated originallywith the oil tank T at the valve body 12 side, is shut off and at thesame time such notches 20 as mentioned above are formed at the throttles2 and 3 on both sides of the shut-off portion, as the one to reduceflowing noises generated during the operation of an eight-equally spacedvalve provided with two throttling units having eight throttles. In thishydraulic pressure control valve, as the notches 20 are provided only ontwo throttles 2, 2 and the throttles 3, 3, a sufficient inclination ingradual increase region can be obtained to satisfy steering feeling inthe case where the depths d of the notches are set to the lower limitvalue 0.04 mm. On the contrary, in the case where this construction isadopted in the manner that six valves are equally spaced, communicationof one or two of three oil grooves 13, 13, 13 communicating with the oiltank T should be shut off therewith, thereby pressure imbalance isbrought about in the radial direction of the valve body 12. In the casewhere the imbalance is larger due to a large steering wheel operation,the relative rotation between the casing 10 and the valve body 12 isobstructed and the hydraulic pressure control valve can't operatenormally.

SUMMARY OF THE INVENTION

The present invention has been devised in consideration of thesecircumstances, and the primary object of the invention is to provide ahydraulic pressure control valve capable of obtaining a desiredinclination (an increasing rate) at gradual increase region of steeringassisting force in the case where the valve is applied to a powersteering apparatus.

Another object of the present invention is to provide a hydraulicpressure control valve in which the relative rotation between a casingand a valve is not obstructed in the case where the valve is applied ina power steering apparatus.

Further object of the invention is to provide a hydraulic pressurecontrol valve capable of realizing a satisfying steering feeling in thecase where the valve is applied to a power steering apparatus.

The hydraulic pressure control valve related to the invention is soconstructed that a valve body of circular section having six oil grooveson its outer periphery at specified intervals is disposed in acylindrical casing having six oil grooves on its inner periphery atspecified intervals, and two throttling units consisting of respectivesix throttles is provided by constructing throttles on both side of eachoil groove, and among six throttles constructing one throttling unit,two throttles facing to each other in the radial direction have regionsin which the throttle areas do not substantially change until therelative angular displacement between the valve body and the casingreaches a predetermined magnitude. Particularly in the hydraulicpressure control valve of the present invention, two throttling unitsare disposed between respective two different destinations and thesupply source of pressure oil and discharge port, thereby pressure oilto the destination is controlled by the variation of the throttle areascreated in both throttling units responsive to the relative angulardisplacement.

The above and further objects and features of the invention will morefully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a conventional hydraulicpressure control valve.

FIG. 2 is a view showing a desired inclement characteristic of steeringassisting force in a power steering apparatus.

FIG. 3 is a view showing a construction mode of a throttle having aregion whose area does not vary.

FIG. 4 is a schematic cross sectional view of a hydraulic pressurecontrol valve related to the invention.

FIG. 5 is a view showing an operation state of a hydraulic pressurecontrol valve related to the invention.

FIG. 6 is an enlarged sectional view showing a construction mode of twothrottles among six throttles.

FIG. 7 is an enlarged sectional view showing a construction mode of therest of four throttles.

FIG. 8 is a view showing variation mode of throttle area of a hydraulicpressure control valve related to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, explanation will be give on the present inventionreferring to drawings showing the embodiments thereof.

FIG. 4 is a schematic cross sectional view of a hydraulic pressurecontrol valve related to the invention applied to a power steeringapparatus.

The hydraulic pressure control valve related to the invention is asix-equally spaced valve comprising the cylindrical casing 10 having sixoil grooves 11, 11 . . . extending in the axial direction on its innerperipheral surface at specified intervals, and the cylindrical valvebody 12 disposed in the casing 10 axially and rotatably and having sixoil grooves 13, 13 . . . extending in the axial direction on its outerperipheral surface at specified intervals. As shown in the figure, thecasing 10 and the valve body 12 are so positioned that the oil grooves11, 11 . . . at the casing 10 side and the oil grooves 13, 13 . . . atthe valve body 12 side are to be stagger-arranged in the peripheraldirection. Between the oil grooves 11, 11 . . . and the oil grooves 13,13 . . . adjacent on both sides thereof in the peripheral direction, twothrottling units comprising respective six throttles 2a through 2f and3a through 3f are formed. The opening of one unit increases as that ofthe other decreases responsive to the relative angular displacementbetween the casing 10 and the valve body 12.

The hydraulic pressure control valve is so applied to a power steeringapparatus that relative angular displacement responsive to the tortionof the tortion bar 4 is to be generated between the casing 10 and thevalve body 12 when steering torque is applied to the steering wheel,like the conventional one, by connecting an input shaft (not shown)which connects with the steering wheel to an output shaft (not shown)which connects with the steering mechanism coaxially through the tortionbar 4, and fixing the casing 10 coaxially on the joint ends of one sideof these shafts, and constructing the valve body 12 integrally on theother joint ends. And as shown in the figure, among six oil grooves 11,11 . . . at the casing 10 side, three oil grooves 11, 11, 11 positioningevery other one are respectively communicated with the oil chambers onboth sides of the hydraulic cylinder S for assisting steering disposedin the steering mechanism. Among six oil grooves 13, 13 . . . at thevalve body 12 side, three oil grooves 13, 13, 13 positioning every otherone are communicated with the discharge side of the hydraulic pump Pbeing pressure oil supply source, and the rest three oil grooves 13, 1313 are respectively communicated with the oil tank T maintained at a lowpressure state. In addition, the tortion bar 4 is positioned coaxiallywith the valve body 12 in the hollow part therein, the annular chamberformed between the outer periphery of the tortion bar 4 and the innerperiphery of the valve body 12 being used as a reflux oil passage to thetank T as shown in the figure.

Both chambers of the hydraulic cylinder S being the destination ofpressure oil are respectively communicated with the hydraulic pump P orthe hydraulic tank T through one throttling unit consisting of sixthrottles 2a through 2f, and are respectively communicate with thehydraulic tank T or the hydraulic tank P through the other throttlingunit consisting of six throttles 3a through 3f. The pressure oilintroduced into the three oil grooves 13, 13, 13 from the hydraulic pumpP flows into the respectively adjacent oil grooves 11, 11 . . . throughthe throttles 2a, 2c, 2e or the throttles 2a, 3c, 3e on both sides ofthe grooves 13, and flows into the rest three oil grooves 13, 13, 13through the throttles 3f, 3b, 3d or the throttles 2b, 2d, 2f on theother side of these oil grooves 11, 11 . . . , and flows back to the oiltank T through a reflux hole opening therein and the annular chamber atthe inner side of the valve body 12.

FIG. 5 shows a state in which steering torque is applied to the steeringwheel and the relative angular displacement is generated between thecasing 10 and the valve body 12, that is, the operating state. In thecase, as shown in the figure, where the throttle areas of the throttles2a through 2f are decreased and those of the throttles 3a through 3f areincreased, when the pressure oil introduced into the oil grooves 13, 13,13 from the hydraulic pump P flows as aforementioned, the conductionresistance at the throttles 2a, 2c, 2e on one sides of the oil grooves13, 13, 13 is larger than that at the throttles on the other sides. Inthis case, between the oil grooves 11, 11, 11 communicating with the oilgrooves 13, 13, 13 through the throttles 2a, 2c, 2e and the oil grooves11, 11, 11 communicating with the oil grooves 13, 13, 13 through thethrottles 3a, 3c, 3e, there is generated a pressure difference, thepressure of the latter being higher than that of the former, thereby thehydraulic cylinder S is to generate steering assisting forcecorresponding to this pressure difference.

On the other hand, in the case where the relative angular displacementbetween the casing 10 and the valve body 12 is opposite to the directionshown in FIG. 5, as the throttle areas of the throttles 2a through 2fare increased and those of the throttles 3a through 3f are decreased, apressure difference whose direction is opposite to the previous case isgenerated between both oil chambers of the hydraulic cylinder S, and thehydraulic cylinder S generates steering assisting force responsive tothis pressure difference.

In this way, the hydraulic pressure control valve related to theinvention changes the throttle areas of the throttles 2a through 2f andthe throttles 3a through 3f responsive to the relative angulardisplacement generated between the casing 10 and the valve body 12 insubstantially proportion to steering torque applied to the steeringwheel, and controls pressure oil to both oil chambers of the hydrauliccylinder S, thereby the hydraulic cylinder S is to generate steeringassisting force. Accordingly, the increment characteristic of steeringassisting force obtained at this time depends upon decreasing mode ofthe throttle areas generated by the throttles 2a through 2f or thethrottles 3a through 3f according to the relative angular displacement.In order to obtain the increment characteristic of steering assistingforce as shown in FIG. 2, that is, two-step characteristic, thedecreasing mode of throttle areas which gradually decreases until therelative angular displacement reaches to a predetermined value andsuddenly decreases after that should be realized, in the throttles 2athrough 2f and the throttles 3a through 3f as aforementioned. And thehydraulic pressure control valve related to the invention ischaracterized by obtaining variation mode of throttle areas asaforementioned in the whole throttling units by making respective twothrottles facing to each other in the radial direction among sixthrottles 2a through 2f or among six throttles 3a through 3fconstructing throttling units be throttles (hereinafter to be calledU-throttles) which have region in which throttle areas do not changesubstantially until aforementioned relative angular displacement reachesa predetermined magnitude and by making the rest four throttles be ones(hereinafter to be called T-throttles) by which throttle areas decreasesubstantially in proportion to the increase of relative angulardisplacement. In FIG. 4 and FIG. 5, the throttles 2a, 3a and thethrottles 2d, 3d respectively facing to each other in the radialdirection are to be U-throttles. The combination of two throttles to beU-throttles may be chosen among any throttles 2a through 2f if only theyface to each other in the radial direction, and it is the same with thecombination among the throttles 3a through 3f. In addition, Combinationsin both throttling units can be different from each other. For example,when the throttles 2a and 2d are to be U-throttles in one throttlingunit, the throttles 3b and 3e are to be U-throttles in the otherthrottling unit, or the throttles 3c and 3f are to be U-throttles.

FIG. 6 and FIG. 7 are enlarged sectional views respectively showingconcrete constructions of a U-throttle and a T-throttle. In addition,each of them is a view in which the peripheral surfaces of the casing 10and the valve body 12 are unfolded on a straight line.

As shown in FIG. 6 at the corner between the inner peripheral surface ofthe casing 10 and the side wall of the oil groove 11, the U-throttle hasthe notch 20 consisting of a first portion 21 crossing the side wall atnearly a right angle and being in parallel with the inner peripheralsurface of the casing 10 and a second portion 22 crossing the portion 21and the inner peripheral surface of the casing 10 respectively atsubstantially a right angle and connecting the both, and at the cornerfacing aforementioned corner and formed between the outer peripheralsurface of the valve body 12 and the side wall of the oil groove 13, theU-throttle has a linear or circular-like notch 23 which chamfers thecorner at a predetermined angle. In addition, the notch 20 issubstantially the same one as shown in FIG. 3. The throttle areas of theU-throttles as maintained substantially constant regardless of theincrement of the relative angular displacement as they are controlled bythe depth d in the radial direction of the first portion 21 at the notch20 until the relative angular displacement is generated between thecasing 10 and the valve body 12 and the valve body 12 reaches theposition at which the side wall of the oil groove 13 agree with the endportion of the notch 20 in the peripheral direction, that is, the valvebody 12 reaches the position shown with a broken line in the figure.After that, until the valve body 12 reaches the position (closureposition) at which the end portion of the notch 20 and the end portionof the notch 23 at the valve body 12 side agrees with each other, asthey are controlled by the depth of the notch 23 successively decreasingin the peripheral direction, they gradually decrease until reachingnearly zero as the relative angular displacement increases.

The T-throttle, as shown in FIG. 7, has a notch 24 only at the corner ofthe valve body 12 side, which chamfers the corner at a predeterminedangle. Thereby, in the T-throttle, immediately after the relativeangular displacement between the casing 10 and the valve body 12 isgenerated, a big variation in the throttle areas is shown, and then,until the end portion at the outer periphery side of the valve body 12of the notch 24 reaches the closure position at which it agrees with thefacing corner of the casing 10, such variation mode of the throttleareas is obtained as gradually decreases at substantially constantchanging rate regardless of the increase of the relative angulardisplacement. In addition, the length in the peripheral direction of thenotch 24 in the T-throttle is made to be nearly same as summation ofthat of the notch 20 and that of the notch 23 in the U-throttle, therebythe U-throttle and the T-throttle can be closed at the same time.

FIG. 8 is a view showing a variation mode of the throttle areas of thewhole throttling units provided with the U-throttles and theT-throttles. As shown with a broken line, in the figure, in theU-throttle, a region where throttle area is constant is obtained in thesphere where relative displacement angle is smaller than 3°. Inaddition, as shown with a dashed line in the figure, in the T-throttle,a variation mode of the throttle areas gradually and proportionallydecreasing in substantially the whole range of relative displacementangles. Here, as aforementioned, there are provided two U-throttles andfour T-throttles and the throttle areas shown with the broken line andthe dashed line show the respective total areas. Accordingly, in thethrottling units consisting of the throttles 2a through 2f or thethrottles 3a through 3f of the hydraulic pressure control valve relatedto the invention, as shown with the solid line in the figure, a gradualdecrease portion is obtained whose throttle area gradually decreasesproportionally to the increase of the relative angular displacement at acorresponding portion to the region where the throttle area is constantin the U-throttle, after the sudden decrease portion of the throttlearea in a region where relative displacement angle is small, the gradualincrease region of steering assisting force as shown in FIG. 2 beingobtained corresponding to the period abovementioned.

The increasing rate of steering assisting force at the gradual increaseregion corresponds to the decreasing rate of the throttle areas at thegradual decrease portion, and the gradual decrease portion of thethrottle areas appears in a region where areas of the U-throttle isconstant. Therefore in order to secure the more increasing rate than apredetermined one of steering assisting force at the gradual increaseregion so as to obtain satisfying steering feeling, it is efficient tomake the depth d small in the radial direction of the notch 20 whichdetermines the throttle area of the U-throttle at the region where thearea is constant. The depth d, however, has a lower limit value due torestriction in processing as aforementioned.

In the hydraulic pressure control valve related to the invention, onlytwo throttles among six throttles 2a through 2f and two throttles amongsix throttles 3a through 3f are the U-throttles, and in the case wherethese throttles are both to have aforesaid lower limit value, they cangive a predetermined inclination to the gradual increase region ofsteering assisting force. In addition, as the U-throttles areconstructed at positions facing to each other in the radial direction,there is no fear that there is generated pressure imbalance in theradial direction. That is, the reason why the number of the U-throttlesare to be two in each throttling unit in the present invention is tosecure the satisfying inclination of steering assisting force in thegradual increase region by making the least number of throttles to beU-throttles under the condition that pressure imbalance is not generatedin the radial direction, and my making the aforementioned constant areaas small as possible according to the depth d of the notch 20 within theallowance in processing. In the hydraulic pressure control valve relatedto the invention, the two step characteristic is realized so thatsatisfying steering feeling can be realized.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within themetes and bounds of the claims, or equivalence of such metes and boundsthereof are therefore intended to be embraced by the claims.

What is claimed is:
 1. A hydraulic pressure control valve, comprising:acasing having six first oil grooves on its inner peripheral surface atspecified intervals; a valve body rotably disposed in said casing andhaving six second oil grooves on its outer peripheral surface atspecified intervals; and two throttling units each of which consists ofsix throttles formed between said first oil groove and second oilgrooves and facing to each other in the radial direction; whereinrespective said two throttling units are disposed between two differentdestinations, and a supply source of pressure oil and discharge port ofpressure oil; and respective two throttles facing to each other in theradial direction in each of said two throttling units, one of whichcommunicates with a discharge port of pressure oil and the other ofwhich communicates with a supply source of pressure oil, are ones whosethrottle areas do not substantially change until the relative angulardisplacement between said casing and said valve body reaches apredetermined magnitude.
 2. A hydraulic pressure control valve as setforth in claim 1, whereinrespective said two throttling units aredisposed between two different destinations, and a supply source ofpressure oil and discharge port of pressure oil.
 3. A hydraulic pressurecontrol valve as set forth in claim 2,wherein said two destinations aretwo oil chambers of a hydraulic cylinder for assisting steering of ahydraulic power steering mechanism.
 4. A hydraulic pressure controlvalve as set forth in claim 1, whereinsaid respective two throttles haverespectively a first notch on a first corner formed between the innerperipheral surface of said casing and the side wall of said first oilgroove, said first notch being provided with a first portion whichcrosses said side wall at substantially right angle and is substantiallyparallel to said inner peripheral surface and with a second portionwhich crosses said first portion and the inner peripheral surface ofsaid casing at substantially right angle thereby to connect the both,and have respectively a second linear or a circular-like notch on asecond corner formed between the outer peripheral surface of said valvebody and the side wall of said second oil groove, said second notchbeing formed by chamfering said second corner at a predetermined angle.5. A hydraulic pressure control valve, comprising:a casing having sixfirst oil grooves on its inner peripheral surface at specifiedintervals; a valve body rotatably disposed in said casing and having sixsecond oil grooves on its outer peripheral surface at specifiedintervals; and two throttling units each of which consists of sixthrottles formed between said first oil groove and second oil groove andfacing to each other in the radial direction;wherein respective said twothrottling units are disposed between two different destinations, and asupply source of pressure oil and discharge port of pressure oil;respective two throttles facing to each other, in the radial directionin each of said two throttling units, one of which communicates with adischarge port of pressure oil and the other of which communicates witha supply source of pressure oil, are ones whose throttle areas do notsubstantially change until the relative angular displacement betweensaid casing and said valve body reaches a predetermined magnitude; andthe respective rest four throttles facing to each other in the radialdirection among respective six throttles constructing said twothrottling units are ones whose throttle areas decrease gradually inrelation to the increase of the relative angular displacement.
 6. Ahydraulic pressure control valve as set forth in claim 5, whereinsaidrespective two throttles have a first notch on the corner formed betweenthe inner peripheral surface of said casing and the side wall of saidfirst oil groove, said first notch being provided with a first portionwhich crosses said side wall at substantially right angle and issubstantially parallel to said inner peripheral surface and with asecond portion which crosses said first portion and the inner peripheralsurface at substantially right angle thereby to connect the both, andhas a second linear or circular-like notch on a corner formed betweenthe outer peripheral surface of said valve body and the side wall ofsaid second oil groove, said second notch being formed by chamferingsaid corner at a predetermined angle, and said respective rest fourthrottles have respectively a third linear or circular-like notch on athird corner formed between the outer peripheral surface of said valvebody and the side wall of said second oil groove, said third notch beingformed by chamfering said third corner at a predetermined angle.
 7. Ahydraulic pressure control valve as set forth in claim 6, whereinthelength in the peripheral direction of said third notch is substantiallysame as summation of that of said first notch and that of said secondnotch.
 8. A hydraulic pressure control valve comprising:a casing havingsix first oil grooves disposed on its inner peripheral surface; a valvebody rotatably disposed in said casing and having six second oil groovesdisposed on its outer peripheral surface; a first throttling unit havingthree radially facing pairs of throttles formed between said first andsecond oil grooves; a second throttling unit having three radiallyfacing pairs of throttles formed between said first and second oilgrooves; wherein, upon rotation of said casing relative to said valvebody in a first direction, said radially facing pairs of throttles insaid first throttle unit increase in throttle area and said radiallyfacing pairs of throttles in said second throttle unit decrease inthrottle area; wherein, upon rotation of said casing relative to saidvalve body in a second direction, said radially facing pairs ofthrottles in said first throttle unit decrease in throttle area and saidradially facing pairs of throttles in said second throttle unit increasein throttle area; wherein said first and second throttle units aredisposed between and in fluid communication with a source of pressureoil and a discharge port; wherein said first and second throttle unitsare in fluid communication with first and second chambers of a hydrauliccylinder so that:when said casing rotates relative to said valve body insaid first direction, said first chamber is in fluid communication withsaid source of pressure oil and said second chamber is in fluidcommunication with said discharge port; and when said casing rotatesrelative to said valve body in said second direction, said first chamberis in fluid communication with said discharge port and said secondchamber is in fluid communication with said source of pressure oil;wherein a selected radially facing pair of throttles in said firstthrottle unit have throttle areas which do not substantially changeuntil said casing rotates relative to said valve body in said seconddirection by a predetermined amount; wherein one of said throttles insaid selected radially facing pair in said first throttle unit fluidlycommunicates with said source of pressure oil and the other of saidthrottles in said selected radially facing pair in said first throttleunit fluidly communicates with said discharge port; wherein a selectedradially facing pair of throttles in said second throttle unit havethrottle areas which do not substantially change until said casingrotates relative to said valve body in said first direction by apredetermined amount; and wherein one of said throttles in said selectedradially facing pair in said second throttle unit fluidly communicateswith said source of pressure oil and the other of said throttles in saidselected radially facing pair in said second throttles unit fluidlycommunicates with said discharge port.
 9. The hydraulic pressure controlvalve according to claim 8 wherein alternating ones of said second oilgrooves each include a first opening in fluid communication with one ofsaid oil pressure source or discharge port, and wherein said casingincludes a second opening facing each second oil groove not having afirst opening located therein, each second opening being in fluidcommunication with the other oil pressure source or discharge port.